1. Field of the Invention
The present invention relates to a data communication apparatus having the function of automatically switching data communication and telephone communication.
2. Related Background Art
One type of conventional facsimile apparatus that has the function of automatically switching facsimile communication and telephone communication periodically sends a quasi-ring back tone to a destination for a time t1 in response to a call from the destination. The apparatus tries to detect a CNG from the destination in a pause period in which the quasi-ring back tone is not sent. In a case where the CNG is not detected in a time t2 elapsed from closing a loop of a telephone line (t2&lt;t1), the apparatus determines the destination to be a telephone, and performs an operator calling operation until the time t1 is over or an operator responds to the operator calling operation. At that time, the apparatus detects a CNG.
As shown in FIG. 11, in a case where the conventional facsimile apparatus does not have a filter in the CNG detecting circuit, and where sent quasi-ring back tones periodically overlap with received CNGs, the apparatus can not detect those CNGs and shift to facsimile communication.
Further, the time t1 is usually set to be about a time T1 (=35 seconds) in the recommendation T.30 of CCITT. Therefore, in a case where a CNG is not detected during the time t2, an operator sometimes cannot respond to an operator calling operation by the apparatus because the period (t1-t2) of the operator calling operation is too short (less than 35 seconds).